Microstructural characteristics and phonon structures in luminescence from surface oxidized Ge nanocrystals embedded in HfO2 matrix

Ge nanocrystals embedded in HfO2 matrices were prepared by rf magnetron sputtering technique. Transmission electron micrographs reveal the formation of spherical shape Ge nanocrystals of 4-6 nm diameters for 800 degrees C and 6-9 nm for 900 degrees C annealed samples. X-ray photoelectron spectroscopy confirms the formation of surface oxidized Ge nanocrystals. Embedded Ge nanocrystals show strong photoluminescence peaks in visible and ultraviolet region even at room temperature. Spectral analysis suggests that emission in 1.58 and 3.18 eV bands originate from T-Sigma(T-Pi) -> S-0, and T-Pi'-> S-0 optical transitions in GeO color centers, respectively, and those in the range 2.0-3.0 eV are related to Ge/O defects at the interface of the oxidized nanocrystals. Temperature dependent photoluminescence study has revealed additional fine structures with lowering of temperature, the origin of which is attributed to the strong coupling of electronic excitations with local vibration of germanium oxides at the surface. (C) 2010 American Institute of Physics. [doi :10.1063/1.3475717]